A drug used to fight one of Britain's fastest growing and most infectious sexually transmitted diseases has been rendered powerless to treat thousands of patients, contributing to the disease's dramatic spread.

The bacteria responsible for gonorrhoea, diagnosed in more than 20,000 people a year, are proving increasingly resistant to the antibiotic ciprofloxacin.

Official figures suggest the treatment is now failing to work properly in one in 10 patients prescribed the drug in England and Wales. Known cases have doubled in five years.

The disease is easily treated in most cases, but many infected people are going undiagnosed and may be contributing to the pace at which resistance is growing.

The drug's failure presents a graphic illustration of how bacteria evolve their defences against therapies. Samples of bacteria taken from patients at 26 clinics and tested with antibiotics suggest ciprofloxacin resistance has more than quadrupled in two years, from just over 2% of patients in 2000 to just under 10% last year.

In Yorkshire and Humberside nearly one in five samples indicated resistance. In London the incidence of resistance was more than 7%, up from less than 1% in 2000. In the south-east generally, one in eight samples showed resistance. In Wales the figure was 11%.

The increases in resistance occurred "irrespective of gender, sexuality and reporting sexual contact abroad", according the the health protection agency (HPA). Its normally sober weekly update on public health matters calls the increase "dramatic".

The update said: "As ciprofloxacin is currently the recommended frontline treatment for uncomplicated gonorrhoea, there is considerable risk of inappropriate management of this infection.

"It is a general principle that the chosen treatment regimes should eliminate infection in at least 95% of patients, and it is now clear ciprofloxacin no longer meets this criterion."

An increasing number of people apparently engaging in high-risk sexual activity are thought to be responsible for the speed at which strains of resistant gonorrhoea bacteria are growing. Kevin Fenton of the HPA suggested such people were not presenting themselves for regular checks and might only attend clinics when they were displaying symptoms.

Ciprofloxacin is routinely given as a single 500mg dose taken orally. Other antibiotics are available, but they are either more expensive or have to be taken intravenously.

In recent years there have been warnings to doctors to prescribe antibiotics only when necessary, and to patients to ensure they complete the treatment. In addition, antibiotics given to livestock for their growth-promotion qualities have been banned for fear that residues passed through food have helped bacteria in humans evolve resistance.

Gonorrhoea strikes particularly hard at women in their late teens and men in their 20s and can cause discharge from sexual organs and a burning sensation when passing urine.

Left untreated it can affect reproductive tubes leading from the ovary and cause ectopic pregnancies. An infected woman can also pass on bacteria during delivery to her baby causing infant conjunctivitis, although this too is treatable by antibiotics.